The present invention relates to systems for detecting distances, and in particular, to a system for avoiding collisions with obstacles. Such a system is applicable to vehicles, robotics, industrial plants and automated guided vehicles (AGVs), for example.
The problem of avoiding collisions in the above mentioned applications is rendered even more complex by the fact that, in the vast majority of cases, operations take place in non-structured environments. Consequently, intelligent control of the vehicles and/or mobile elements involved must plan for anti-collision strategies and sensing systems that are able, for instance, to plan paths that are alternative to the ones defined in the initial control step and/or are able to signal the presence of obstacles, in particular, based upon data coming from different sensors associated with the vehicle/mobile element.
Typically, sensors are used that are able to measure the so-called time of flight (TOF). The time of flight is the time interval that elapses between the transmission of a wave and the reception of the wave reflected after interaction with an obstacle.
Implementation of driving schemes based upon the use of distance sensors that are able to detect and/or measure the time of flight usually make use of algorithms that handle the transmission intervals of the various sensors. For example, robotics and vehicles make use of these driving schemes. The vehicle/mobile element is usually equipped with a number of sensors which act in a number of directions in order to examine the entire surrounding space, or at least, the portion of space of interest for avoiding collisions. The above approach requires a dedicated control unit, such as the one described, for example, in Sensors for Mobile Robots, Theory and Application, by H. R. Everett, A. K. Peters Ltd., Natick, Mass., 1995.
A number of sensors associated with the same vehicle/mobile element may interact together, thus giving rise to situations of interference that may reduce the quality of the measurements. In addition, the use of a number of vehicles/mobile elements operating in a multi-user scenario introduces an additional source of interference.
The algorithms according to the prior art are very difficult to apply to situations in which a number of vehicles/mobile elements are present. In this case, there is an unavoidable need to provide a monitoring and coordinating unit for the entire vehicles/mobile elements involved. This results, in a majority of the applications, in additional costs and in more general terms, a considerable loss in the autonomy of movement of vehicles/elements designed to operate independently from one another.
In view of the foregoing background, an object of the present invention is to provide a system for detecting distances which can be applied to systems comprising a number of vehicles and/or mobile elements. The system operates in a multi-user scenario, namely, in a context in which each sensor encounters some difficulty in distinguishing its own echo on account of the signals generated simultaneously by the other sensors associated with the same vehicle/mobile element or else with different vehicles/elements.
The system according to the present invention addresses the problem linked to multipath-fading, i.e., where a sensor is not able to distinguish the echo reflected by the obstacle along the shortest path (a path representing the actual distance from the obstacle) from the other echoes that cover longer paths, such as the ones generated by indirect reflections due to walls or other obstacles.
According to the present invention, the above objective is achieved by a system comprising at least one transducer for generating a transmission signal to be sent in a direction of the obstacle, and for obtaining a receiving signal corresponding to an echo produced by a reflection of the transmission signal off the obstacle. The system further comprises a chaos generator for driving the transducer so that the transmission signal has characteristics of a chaotic signal.
In particular, the invention exploits the possibility of using the advantages afforded by chaotic communications in order to address the problems linked to the use of sensors based on the measurement of the time of flight (TOF).
Chaos-based communication schemes exploit the high sensitivity to variations in the initial conditions, and in the values of the system parameters for generating a large variety of codes based upon non-predictable sequences. As is known, chaotic communication schemes may be divided into coherent schemes and non-coherent schemes, according to whether the receiver knows or does not know the carrier transmitted.
For a general overview on chaotic communication schemes, reference may be made to the following works by G. Kolumbàn et al.: xe2x80x9cThe Role of Synchronization in Digital Communications using Chaosxe2x80x9d, Part I: Fundamentals of Digital Communications, IEEE Trans., CAS-1, October 1997; Part II: Chaotic Modulation and Chaotic Synchronization, IEEE Trans. CAS-1, November 1998; Part III: Performance Bounds for Correlation Receivers, IEEE Trans. CAS-1, December 2000.
In particular, in the currently preferred embodiment, the approach according to the invention makes use of a chaotic pulse position modulation (CPPM) scheme. According to this scheme, modulation of the information is contained in the time interval elapsing between the generation of two successive pulses. The interval is determined by a chaotic digital source. Useful reference may also be made to G. M. Maggio et al., xe2x80x9cChaotic pulse-position modulation for ultrawide-band communication systemsxe2x80x9d, by G. M. Maggio et al., Proc. UWB ""99, Washington D.C., Sept. 28-30, 1999.
A signal is preferably sent to the sensor, which includes pulses generated according to a CPPM scheme of an analog type, such as the one described in xe2x80x9cChaos Preservation through Continuous Chaotic Pulse Position Modulationxe2x80x9d, by L. Fortuna, M. Frasca, A. Rizzo, ISCAS 2001.
The advantage of using an analog scheme, with regards to generation of the sequence of time intervals necessary for modulation, results primarily in a considerable savings. According to the traditional CPPM scheme for generating a chaotic sequence, it is required for implementing a chaotic map on a microprocessor provided with a sufficiently high number of bits. Instead, recourse to an analog or continuous CPPM scheme allows the use of circuitry with just a few operational amplifiers and a small number of other discrete components which are not costly at all.
The distance from the obstacle is evaluated by performing a correlation between the signal transmitted and the signal received, which presents only one important peak at TOF. This property is provided in multi-user scenarios with respect to correlating between two chaotic signals coming from different sources or between two different portions of the same chaotic signal. In addition, the problem linked to multipath fading is overcome, in so far as the correlation peak due to the wave reflected directly is much higher than other possible peaks due to multiple reflections of the same signal.
The present invention is therefore characterized by a low cost. The system does not require dedicated hardware and can be implemented using low-cost circuitry. In terms of performance, the system according to the invention addresses the problems that arise in multi-user scenarios with multipath fading without deteriorating the original performance of the sensor used. In terms of flexibility, the system drives a wide variety of sensors. Moreover, this is possible to do without a central control unit. Operation of the sensor is managed at a local level, and in all the applications in which a plurality of sensors are to be installed, there is no need to have a control or coordination unit.